This invention relates to means for effecting a seal between surfaces, and more particularly to means for establishing a fire resistant metal-to-metal seal between the outer surface of a tubular member, such as a tension hanger, and the inner surface of a mating member of the type that are known to exist in wellhead equipment.
The fact that extreme service conditions are encountered in wellhead applications has long been recognized. Moreover, it has long been known that the nature of such extreme service conditions encompasses, by way of example and not limitation, conditions such as the presence of high and low temperature, sour gas, high fluid velocity, pressure cycling, thermal shock, and/or the existence of forces of vibration, bending, compression, tension or any combination of these forces. In an effort to provide equipment that would be suitable for employment in such wellhead applications, i.e., that would successfully withstand being subjected to extreme service conditions of this type, metal-to-metal seals have heretofore been employed for purposes of effectuating seals in equipment designed to be used in wellhead applications of the aforedescribed type. This selection of metal-to-metal seals for use in this manner has been influenced to some extent by environmental and economic considerations. Moreover, the metal-to-metal seals that have actually been selected for use for this purpose have been of various designs. By way of illustration, reference may be had among others, to U.S. Pat. No. 4,390,186, which issued on June 28, 1983 to John K. McGee et al., for a showing of a metal-to-metal seal that is disclosed to be suitable for use in equipment, which is designed for employment in wellhead applications.
Although these earlier types of metal-to-metal seals when employed in equipment designed for use in wellhead applications, have proven generally to be capable of withstanding the extreme service conditions associated with such applications, i.e., conditions of a sort that have been enumerated hereinbefore, these metal-to-metal seals were never intended to be fire resistant. That is, no requirement existed insofar as the design of these metal-to-metal seals was concerned that they embody the capability of maintaining sealability during periods of thermal expansion and contraction occasioned by the occurrence of wellhead fires. It is only more recently that the matter of fire resistance has come to be viewed as a consideration in the design of seals of the type found in equipment that is intended for use in wellhead applications. Moreover, to some in the industry this matter of fire resistance has gone beyond the state of being simply a consideration, but rather has now risen to the level of being a requirement that future designs of metal-to-metal seals must satisfy.
The high temperatures which are encountered during wellhead fires give rise to a variety of problems. Included among these are problems that can be linked to the rapid thermal heatup and cooldown of the material which is exposed to the wellhead fire, the expansion and/or contraction of the exposed material, and/or a loss in the properties which the exposed material exhibits. For ease of classification, however, the aforereferenced problems fall basically into two categories. Namely, there are those problems which relate to the structural characteristics exhibited by the wellhead equipment material upon being exposed to a wellhead fire, and there are those problems which relate to the capability of seals in wellhead equipment to maintain their sealability when the wellhead equipment is subjected to a wellhead fire.
Regarding first the matter of the structural characteristics of wellhead equipment material, insofar as rendering such material fire resistant is concerned the loss of tensile strength exhibited thereby when exposed to a wellhead fire can be compensated for in several ways. For example, the pressure limits which the equipment must be capable of withstanding is commonly permitted to be downrated by up to twenty-five percent. In addition, the pressure vessel walls of the equipment in question generally are permitted to be constructed such that they are oversized. Accordingly, it has been found that this twenty-five percent downrating of the pressure limits which the equipment must be capable of withstanding coupled with the oversizing of the pressure vessel walls of the equipment is sufficient to compensate for the loss of the tensile strength that occurs when the wellhead equipment is exposed to elevated temperatures such as those to which this equipment is exposed during the occurence of a wellhead fire.
Although wellhead housings become large when the walls thereof are oversized, such housings nevertheless remain within practical limits. Therefore, there is no necessity to make use of exotic steels, etc., for this type of equipment. This is not to say, though, that future developments in the area of material research may not produce new cost effective, high strength alloys, which would enable a reduction to be had in the sizing of wellheads of the type to which reference has been had hereinbefore.
Turning now to the matter of the sealability of the seals that are embodied in wellhead equipment, it is essential for the reasons that have been discussed previously herein that such seals be effectuated through the use of metal-to-metal seals. On the other hand, however, if such metal-to-metal seals are to be capable of exhibiting adequate tensile strength at elevated temperatures the view has been taken that there must be utilized therein high strength materials as overlays or seal ring materials. Elastomers, as they are known today, are known to perform unsatisfactorily when employed under the sort of conditions to which wellhead equipment is subjected when a wellhead fire occurs. The one nonmetallic material which may have some merit for use in such applications is that which is referred to by those in this industry as "Grafoil".
By and large, therefore, it can thus be seen that in order to develop wellhead equipment that is fire resistant, a need has existed to develop improved sealing techniques that would be suitable for use to effect seals that would maintain their sealability at elevated temperatures. More specifically, there has existed a need to develop improved high temperature sealing techniques that would be applicable for use in connection with both the tubular and annular seals that are to be found in wellhead equipment, and which would enable the latter equipment to withstand in terms of sealability the range of temperatures to which such equipment would commonly be exposed in the course of a wellhead fire. In this context, in order to develop such an improved high temperature sealing technique there would exist a need to address the following areas: the thermal and metallurgical characteristics of the materials involved, the relative movement that occurs during the mating parts, and the sliding action that the seal must endure.
Efforts have been undertaken looking to the development of such high temperature sealing techniques. Moreover, the focus of these efforts, at least at the outset, has been directed towards successfully providing a sealing means wherein the seal effected therewith would be capable of withstanding the high temperature required in order to enable the sealing means to be classified as being fire resistant. To this end, considerable time and effort was devoted to the development of a suitable clamp connection that would maintain its sealability at elevated temperatures. However, not only did the mass of such a clamp connection prove to be detrimental to heat exchange properties of the wellhead equipment per se, but indeed proved to be uncontrollable in terms of torsional deflection and permanent set. In turn, the latter prevented retention of any seal that was dependent upon the clamp connection as a holding device.
As the result of the realization of the above, the development of a studded clamp connection was undertaken. However, the unfavorable heat transfer properties of the added mass of the clamp soon led to the abandonment of the clamp itself. This was done principally so that a more favorable heat transfer condition could be realized in a less irregular surface surrounding the wellhead housing. It was then concluded that the context of attempting to render wellhead equipment fire resistant clamp connections should not be utilized. Consequently, efforts were directed towards providing a new and improved form of connection. These efforts led to the development of the connections which comprise the subject matter of the five co-pending patent applications to which reference has been had hereinbefore, and which can be found listed herein under the heading "Cross Reference To Related Applications".
Apart from the need to provide fire resistant connections, a need has also been evidenced for accomplishing in a wellhead assembly a fire resistant seal as between, for instance, a tubular member such as a tension hanger and a mating member with which the tubular member is intended to be cooperatively associated. Insofar as, for example, tension hangers are concerned any seal utilized therewith must be capable of maintaining its sealability while yet undergoing movement of both an axial and a radial nature. More specifically, in accord with the mode of operation of a tension hanger, the latter is designed to be inserted into the bore of a mating member, such as a tubing bonnet. Moreover, in the course of being so inserted into the tubing bonnet, the seal with which the tension hanger is suitably provided is energized by virtue of its being moved into a tapered area suitably formed for this purpose in the mating member. That is, the effect of moving the seal into the aforesaid tapered area is to accomplish a preloading of the seal. After being preloaded in the aforesaid manner, movement is had of the seal into the straight bore of the mating member. The sidewalls defining the straight bore function to retain the seal preloaded for purposes of achieving both proper bearing stress and sealability. However, the exact location of the seal from an axial standpoint relative to the straight bore varies as a function of the extent to which the tension hanger must be moved in an axial direction in order to achieve the requisite tensioning thereof.
From the preceeding discussion, it can thus be seen in effecting a seal between a tension hanger and a mating member, the seal is first subjected to radial movement in order to accomplish the preloading thereof. Thereafter, the seal is subjected to axial movement while the tension hanger is undergoing tensioning. Having met these requirements, a seal in order to be considered fire resistant must also embody the capability of being able to maintain its sealability while being subjected to the elevated temperatures which are known to prevail during the occurrence of a wellhead fire. The effect, insofar as sealability is concerned, of these elevated temperatures on the tension hanger and the mating member that is cooperatively associated therewith is to cause an expansion and/or contraction of the exposed material of these members. In this regard, movement, i.e., expansion and contraction, in an axial direction is of primary concern. Movement in a radial direction generally poses no significant problem inasmuch as the coefficient of expansion of the two members, i.e., the tension hanger and the mating member, can be preselected such that they are very similar whereby any differential radial movement between the tension hanger and the mating member is insufficient to overcome the springing action of the radially compressed seal member, i.e., the preloading to which the seal has been subjected as described previously herein. On the other hand, during the course of moving axially within the straight bore of the mating member, the seal must maintain its sealability as the tension hanger and mating member expand and/or contract in response to their being subjected to elevated temperatures occasioned by the occurrence of a wellhead fire.
Thus, to summarize, it has been concluded from analytical and test results that materials do exist which are suitable for use in forming pressure containing members of wellhead housings, valve bodies, and bonnets. Further, it is viewed as being practical to construct valve bodies and wellhead housings of such materials. That is, the use of such materials for this purpose does not lead to enormous enlargement of the equipment to the point of being impractical. On the other hand, however, such materials are disadvantageously characterized insofar as concerns their suitability for use in performing a sealing function. Accordingly, it is essential that a new and improved form of seal be developed for use within wellhead equipment housings. Furthermore, such a new and improved form of seal must be of sufficient size and integrity to withstand the loading forces necessary to effect the sealing function. In addition, the materials from which such seals are fabricated must of necessity be selected for compatibility for their elevated temperature strength, and their thermal conductivity. Namely, it is very important that the material selected for use for fabricating such seals be such that the mating sealing surfaces that are produced as a consequence of the use thereof are compatible from the standpoint of thermal expansion and contraction, corrosivity, weldability and gall resistance. However, even when the above criteria has been satisfied, therre still remains a need to provide a high temperature seal, which in terms of its design as contrasted to the matter of the materials from which it is formed, is suitable for use in wellhead equipment that may be subjected to elevated temperatures of the type that are experienced during the course of a wellhead fire. That is, a need has been evidenced for a seal design wherein a seal constructed in accordance therewith would when employed in wellhead equipment be characterized by the fact that it possesses the capability of maintaining its sealability, even when the wellhead equipment in which it is embodied is involved in a wellhead fire.
It is, therefore, an object of the present invention to provide a new and improved form of metal-to-metal seal suitable for employment in wellhead equipment.
It is another object of the present invention to provide such a metal-to-metal seal, which when employed in wellhead equipment is capable of withstanding the conditions imposed thereupon during the occurrence of a wellhead fire.
It is still another object of the present invention to provide such a fire resistant metal-to-metal seal, which is characterized in that it exhibits adequate tensile strength even at the elevated temperatures that exist when a wellhead fire occurs.
A further object of the present invention is to provide such a fire resistant metal-to-metal seal, which is characterized in that it exhibits a capability of being able to maintain its sealability even at the elevated temperatures that exist when a wellhead fire occurs.
A still further object of the present invention is to provide such a fire resistant metal-to-metal seal, which is particularly suited for embodiment in a tubular member of the sort that is intended for employment in a wellhead assembly.
Yet an object of the present invention is to provide such a tubular member embodying such a fire resistant metal-to-metal seal wherein the seal is intended to be made to undergo radial movement in order to accomplish the preloading thereof.
Yet another object of the present invention is to provide such a tubular member embodying such a fire resistant metal-to-metal seal wherein after being preloaded the seal is intended to be capable of undergoing axial movement while yet retaining the bearing stress and the sealability required thereof.
Yet still another object of the present invention is to provide such a tubular member embodying such a fire resistant metal-to-metal seal which is relatively inexpensive to provide and easy to employ, while yet being capable of providing reliable and effective service even when exposed to the conditions that exist when a wellhead fire occurs.